The present invention relates generally to the recovery of metal ions from aqueous media. More particularly, the present invention relates in one embodiment to an ion exchange resin, in another embodiment to a process for removing iron(III) cations from an aqueous medium containing sulfuric acid and other polyvalent metal cations using that ion exchange resin, and in a still further embodiment to a generalized process for removing polyvalent metal ions from aqueous acid solution.
Removal of radionuclides and other heavy metal ions from aqueous solutions has been the subject of extensive research. One of the areas in which this research is primarily focused is removing heavy metal ions from aqueous solutions through selective complexation.
Selective complexation is typically performed using ligands polymerized on polymer supports. Chang et al. Talanta 42:1127 (1995) describe using immobilized imidazolines for trace metal recovery. Tomita et al. J. Poly. Sci., Poly. Chem. Ed. 34:271 (1996) discuss using immobilized kojic acid for trace metal recovery. Lan et al. Anal Chim. Acta 287:101 (1994) teach using immobilized quinolinol for trace metal recovery. Buchanan et al. Can. J. Chem. 69:702 (1991) describe using immobilized crown ethers for trace metal recovery. Kawamura et al. Ind. Eng. Chem. Res. 32:386 (1993) disclose using immobilized polyethylenimines for trace metal recovery. Van Berkel et al. Europ. Poly. J. 28:747 (1992) discuss using immobilized pyrazoles for trace metal recovery. Kamble et al. J. Appl. Poly. Sci. 56:1519 (1995) teach using immobilized oximes for trace metal recovery. Lezzi et al. J. Appl. Poly. Sci. 54:889 (1994) discuss using immobilized dithiocarbamates for trace metal recovery.
Ion exchange resins with phosphorous-containing ligands are an important group of metal ion chelating agents. The selectivity of these types of ligands can be varied by changing the structure of the phosphorous ligand. The ability of these ligands to strongly coordinate different metal ions leads to significant levels of ionic complexation under highly acidic conditions.
Horwitz et al. Solv. Extr. Ion Exch. 11:943 (1993) have shown that immobilized diphosphonic acid groups have very high affinities for a series of metal ions because of the coordinating ability of the phosphoryl oxygen and a ligand structure that permits chelation of the metal ions. High loadings at equilibrium are attained under contact times on the order of days unless the phenyl rings within the polymer are sulfonated, which reduces the equilibration time to on the order of ten minutes. Chiarizia et al. Solv. Extr. Ion Exch. 12:211 (1994).
The introduction of bifunctionality into ion exchange resins has been discussed as a coupling of an access mechanism (permitting all ions into the matrix rapidly) with a recognition mechanism (a second ligand selectively complexes a targeted metal ion). Alexandratos et al. Macromolecules 21:2905 (1988). Studies with diphosphonate-immobilized polymer have shown that both ligands on the resin complex far greater levels of metal ions than either one could alone. Alexandratos et al. Macromolecules 29:1021 (1996)
The access mechanism introduced by the sulfonic acid ligand is due to the ligand""s hydrophilicity that permits rapid entry of metal ions into the matrix. It has been found that monofunctional phosphonic acid microporous resin cross-linked with 2 percent divinylbenzene (hereinafter xe2x80x9cDVBxe2x80x9d) lost most of its ability to complex Eu(III) from 1 N HNO3 compared to the performance of this material in 0.04 N acid. Trochimczuk et al. J. Appl. Poly. Sci. 52:1273 (1994). These results were attributed to a collapse of the microporous structure in high ionic strength solutions that restricts access.
Trochimczuk et al., above, describe that linking sulfonic acid groups and phosphonic acid groups to different phenyl rings increases the amount of Eu(III) complexed from high ionic strength solutions. The results suggested an increased access of the metal ions into the polymer matrix coupled with increased complexation by the phosphonate ligands. However, the advantage of increased complexation was offset by the decreased resin capacity from the lower level of substitution necessitated by the copolymerization with styrene. In addition, when a cross-linked phosphonate polymer that was not copolymerized with styrene was sulfonated, a relatively small metal binding capacity was again observed in 1 N nitric acid.
Copper metal is obtained from copper ores by several well-known processes. One of the most frequently used processes is referred to as a solvent extraction-electrowinning (SX-EW) process in which copper(II) ions are first leached from the ore using sulfuric acid followed by extraction with a kerosene-based copper-specific solvent mixture. The copper ions are then stripped from the solvent mixture using a copper sulfate-sulfuric acid electrolyte solution (CuSO4xe2x80x94H2SO4 electrolyte solution). The copper recovery process is then completed by electrowinning of copper from the copper-enriched strip solution.
Small amounts of iron(II) and iron(III) cations are commonly transferred with the copper cations to the electrowinning solution. Iron transfer occurs by chemical co-extraction (binding to the oxime molecule) and by entrainment of iron-containing aqueous solution in the copper-loaded organic solution. As copper is depleted from the CuSO4xe2x80x94H2SO4 electrolyte solution during copper electrowinning (EW), the concentration of iron in solution increases. This build up of iron in solution results in a loss of current efficiency in the electrowinning process due to a continuous oxidation/reduction of Fe2+/Fe3+. That loss of current efficiency can amount to about 2-3 percent per gram of iron in solution. The conventional treatment technique for iron control has been to periodically bleed or purge a portion of the iron-rich, copper-depleted electrolyte and replace it with a sulfuric acid electrolyte solution.
In a copper electrowinning process, lead-based alloys are used as oxygen-evolving anodes. Soluble cobalt(II) (50 200 ppm) ions are added to the aqueous sulfuric acid copper-containing electrolyte to control corrosion of the lead anode, and to prevent xe2x80x9cspallingxe2x80x9d and possible lead contamination of the copper cathode. During bleed of the spent (copper-depleted) electrolyte to control iron concentration, cobalt is lost from the system. Cobalt must be continually added to the electrowinning electrolyte to make up cobalt lost through the bleed stream. Cobalt replacement to control lead anode corrosion is a major operating expense in copper SX-EW plants. Removal of the iron from the electrowinning electrolyte solution while retaining the cobalt is desired.
Sulfonic acid functional group cation exchange resins are widely used in the water treatment industry and other industrial processes for the removal of cations, such as iron, from aqueous process streams. Such resins also bind and accumulate other cations, such as calcium, magnesium, and sodium, that are undesirable in an iron removal process, necessitating frequent regeneration of the resin.
Gula et al., U.S. Pat. No. 5,582,737, the disclosures of which are incorporated herein by reference, describe a process that separates and removes iron(III) from aqueous sulfuric acid solution containing additional metal ions such as copper and cobalt ions as are found in depleted copper electrowinning electrolyte solutions. That process utilizes gem-diphosphonic acid ion exchange particles that are preferably also sulfonated to remove the iron(III) ions, while permitting (1) copper, cobalt and other mono- and divalent metal ions to be recycled into the copper electroplating recovery process, thereby saving on the costs of cobalt that would otherwise be discarded, and (2) regeneration of the ion exchange particles for further use and recycle to the separation and removal steps.
The process for regenerating the gem-diphosphonic acid ion exchange particles used in the above process disclosed by Gula et al. involves use of sulfurous acid (H2SO3) to reduce the bound iron(III) ions to iron(II) ions that are free in solution. The sulfurous acid is usually generated prior to the iron(III) reduction step by sparging an aqueous solution with SO2 gas, which dissolves to form H2SO3. The use of SO2 gas in the Gula et al. regeneration process raises issues relating to the availability of SO2, the costs of the sulfur dioxide storage and delivery systems, and pressurization of the system needed to maintain SO2 dissolution.
Gula et al. disclose that in their regeneration process, the addition of at least a catalytic amount of copper ions was found to increase the efficiency of SO2-caused regeneration. The catalytic amount of copper ions could be added to the copper electrowinning bleed solution itself, or could be provided, for example as a copper sulfate solution prepared expressly for this purpose. Alternatively, a solution of sulfuric acid (H2SO4) containing copper(II) ions could be passed over copper metal and then sparged with SO2 gas to form the sulfurous acid solution containing a catalytic amount of copper(I).
Another process for regenerating geminal diphosphonate iron(III)-bound ion exchange particles is disclosed in allowed Dreisinger et al. U.S. patent application Ser. No. 09/019,677 filed on Feb. 6, 1998. In that process, the iron(III)-bound ion exchange particles are contacted with an aqueous SO2-free reducing solution containing 0.1 to about 6 molar sulfuric acid and an amount of copper(I) ions sufficient to reduce the solid phase-bound iron(III) ions to iron(II).
The gem-diphosphonic acid ion exchange particles used by Gula et al. and Dreisinger et al. have a high capacity, but are relatively expensive and difficult to prepare. The monophosphonic acid ion exchange particles of Trochimczuk et al. are more readily prepared and less expensive than are those of Gula et al., but have reduced capacity for polyvalent cations. It would be beneficial if a monophosphonate ion exchange resin could be prepared that exhibited a high polyvalent metal cation capacity in 1-4 N nitric acid or 1-2 N sulfuric acid similar to that exhibited by the more expensive and difficultly prepared diphosphonate ion exchange resins. It would also be beneficial if such a monophosphonic acid ion exchange resin could be used in a process for iron(III) removal from sulfuric acid-containing aqueous media such as those utilized by Gula et al. and Dreisinger et al. The disclosure that follows illustrates one such material and its use in removing heavy metal ions from aqueous acid solutions and particularly, in a process for iron(III) removal from sulfuric acid-containing aqueous media that also contain other polyvalent metal ions.
In one embodiment, the present invention relates to an ion exchange resin that is useful, inter alia, for removing polyvalent heavy metal cations from an aqueous solution. A contemplated ion exchange resin is a cross-linked water-insoluble polymer comprised of polymerized monomers having a phenyl ring. At least 50 mole percent of the polymerized monomers are phenyl ring-containing monomers that have a phosphonic acid ligand linked thereto. At least 35 mole percent of the polymerized monomers are phenyl ring-containing monomers having both a linked phosphonic acid ligand and a linked sulfonic acid ligand, with the remaining monomers being free of sulfonation. A contemplated monophosphonic/sulfonic acid resin of this embodiment typically contains about 2 to about 5 millimoles of phosphorus per gram (mmol/g) of polymer and has a ratio of millimoles of phosphorus (phosphonate) to millimoles of sulfur (sulfonate) of up to 3:1, and preferably 3:1 to about 1:2.
In a second embodiment, the present invention relates to an improved process for the separation and removal of iron(III) (Fe3+) cations (ions) from aqueous metal cation-containing acid solutions, such as a sulfuric acid solution. In accordance with this embodiment, a contemplated process comprises the following steps:
(a) An aqueous metal ion-containing sulfuric acid solution that contains iron(III) ions as well as ions having a valence of less than +3 of at least one additional metal having a valence of +2 is contacted with solid ion exchange medium that is preferably in the form of particles. The ion exchange medium binds to the iron(III) ions in preference to the additional metal ions present to form a solid/liquid phase admixture. A contemplated ion exchange resin is a cross-linked water-insoluble polymer comprised of polymerized monomers that contain monophosphorus acid functional group ligands and also contain sulfonic acid functional groups. A monophosphorus acid functional group contains a single phosphorus atom that can be present in the form of a phosphonic acid as in the above embodiment, a phosphinic acid group or a phosphoric acid ester. A cross-linked sulfonated phosphorus acid functional group-containing polymer is referred to herein as a monophosphorus/sulfonic acid resin or ion exchange resin. The polymerized monomers preferably include a phenyl ring to which the monophosphorus acid functional group is bonded. A monophosphorus/sulfonic acid resin contemplated in this embodiment contains about 2 to about 5 millimoles of phosphorus per gram (mmol/g) of polymer and has a ratio of mmol/g of phosphorus (phosphonate or phosphonic or acid, phosphinate or phosphinic acid, or phosphate or phosphoric acid) to mmol/g of sulfur (sulfonate or sulfonic acid) of about 4:1 to about 1:2. A monophosphorus/sulfonic acid ion exchange resin of the first embodiment is a particularly preferred polymer for use in this embodiment.
(b) The contact is maintained between the sulfuric acid solution containing iron(III) ions and a sufficient amount of solid ion exchange particles for a time period sufficient to form solid phase-bound iron(III) ions and an aqueous liquid phase containing sulfuric acid and the additional metal ions, as well as a lower concentration of iron(III) ions.
(c) The solid and liquid phases are separated.
(d) The separated solid phase-bound iron(III) ions are contacted with an aqueous stripping solution, thereby forming a second solid/liquid phase admixture.
(e) The second solid/liquid phase admixture is maintained at a temperature of about room temperature to about 95xc2x0 C. for a time period sufficient to form an aqueous liquid phase containing iron(II) cations and a solid phase of regenerated ion exchange particles.
(f) The iron-containing liquid phase is separated from the regenerated solid phase ion exchange particles.
In one aspect of this embodiment of the invention, the aqueous stripping solution contains 0.1 to about 6 molar aqueous sulfuric acid and an amount of reductant sufficient to reduce the solid phase-bound iron(III) ions to iron(II) ions. In one particular aspect of this embodiment, the stripping solution is free of added SO2 or H2SO3 and the reductant is a copper(I) ion-containing aqueous reducing solution prepared by dissolving copper(O) in a 0.1 to about 6 molar aqueous sulfuric acid solution. Alternatively, a copper(I) salt is dissolved directly in a 0.1 to about 6 molar aqueous sulfuric acid solution.
In another particular aspect of this embodiment of the invention, the 0.1 to about 6 molar aqueous sulfuric acid solution used to make the copper(I) ion-containing aqueous reducing solution is free of added SO2 or H2SO3 and is a spent electrolyte solution from a solvent extraction copper electrowinning process.
In yet another particular aspect of this embodiment of the invention, the 0.1 to about 6 molar aqueous sulfuric acid solution used to make the copper(I) aqueous reducing solution is free of added SO2 or H2SO3 and is recycled from an ion exchange medium regeneration process, and already contains some iron(II) ions.
In a still further particular aspect of embodiment of the invention, the separated solid phase-bound iron(III) ions are contacted with an aqueous reducing solution containing 0.5 to about 6 molar sulfuric acid, at least a catalytic amount of copper ions and an amount of sulfurous acid dissolved SO2 sufficient to reduce the solid phase-bound iron(III) ions to iron(II) ions to form a second solid/liquid phase admixture.
In another aspect of this embodiment of the invention, the stripping solution is about 4 to about 10 M hydrochloric acid.
A more general process for removing polyvalent metal cations having a valence of +3 or more from an aqueous acid solution; i.e., a solution having a pH value less than about 7 constitutes another embodiment of the invention.
That process comprises the steps of (a) forming a solid/liquid phase composition by contacting an aqueous solution containing polyvalent metal cations having a valence of +3 or more with a solid ion exchange medium. That ion exchange medium is a water-insoluble resin that is comprised of: (i) polymerized phenyl ring-containing monomers of which at least 50 mole percent have a phosphonic acid ligand linked to the phenyl ring via (through or by means of) a methylene group (xe2x80x94CH2xe2x80x94); (ii) those phosphonic acid ligands providing the resin with about 2 to about 5 millimoles per gram (mmol/g) of phosphorus, and (iii) a sufficient amount, preferably at least 35 mole percent, of sulfonic acid ligands linked to the phenyl rings such that the ratio of mmol/g of phosphonic acid to mmol/g sulfonic acid in the resin is up to 3:1.
(b) That contact is maintained for a time period sufficient for the ion exchange medium to bind the polyvalent metal cations and form solid phase-bound metal cations and a liquid phase from which polyvalent metal ions have been removed; i.e., a liquid phase having a lower concentration of polyvalent metal ions than that used in contacting step (a).
(c) The solid and liquid phases are then separated.
It is preferred that at least 50 mole percent of the polymerized phenyl ring-containing monomers have both a phosphonic acid ligand and the sulfonic acid ligand linked thereto. It is also preferred that the ratio of sulfonic acid capacity to phosphonic acid capacity of the ion exchange resin be about 1:6 to about 1:1. The valence of the polyvalent metal cation removed with this process is preferably +3, and the process is preferably carried out as a pH value of 1 or below.
The present invention has several benefits and advantages. One benefit is that the resin matrix of the present invention remains hydrated in high ionic strength solutions.
Another advantage of the ion exchange resin of the present invention is that the resin matrix permits metal ions to be rapidly complexed.
Yet another benefit is that a contemplated ion exchange resin is less expensively and more readily prepared than is the sulfonated gem-diphosphonic acid resin used in Gula et al. U.S. Pat. No. 5,582,737.
A still further advantage of the present invention is that the iron(III) capacity of a contemplated ion exchange resin is unexpectedly high compared to a gem-diphosphonic acid resin used by Gula et al.
A still further benefit of the present invention is that a contemplated ion exchange resin is readily prepared.
Still further benefits and advantages of the present invention will be apparent to a person of ordinary skill from the description that follows.